The purpose of this study is to assess the safety, tolerability, and immunogenicity of booster dose of vaccine in participants who are generally healthy or with stable pre-existing health conditions. Study details include:

• The study duration per participant will be approximately 209 days (28 days screening, 1 day vaccination, and 180 days follow-up).
• The treatment will include 1 booster dose only.
• The visit frequency will be 6 on-site visits and 1 phone visit.

This is a Phase II, prospective, randomized, observer-blinded, multi-center study, to evaluate the safety, tolerability, and immunogenicity of a booster vaccination with AZD1222, mRNA-1273, or MVC-COV1901 vaccine. Approximately 960 participants aged 18 ~ < 80 years, who received homologous two doses of vaccines 150 ~ 365 days ago, will be enrolled and divided into three groups. Each group will consist of 320 eligible subjects, and for each group the randomization will be stratified according to study site and age to four treatments (AZD1222, half dose of mRNA-1273, full dose or half dose of MVC-COV1901 in 1:1:1:1 ratio). Therefore, within a group, for either age stratum, there will be at least 30 participants for each treatment.

Inclusion Criteria:

1. Male or female participants aged ≥ 18 years at randomization.

2. Healthy adults or adults with pre-existing medical conditions who are in stable condition. A stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease 3 months before enrollment and expected to remain stable for the duration of the study.

3. Documented to have received two homologous doses of AZD1222, mRNA-1273, or MVC-COV1901 vaccine, with the latest dose between 150 and 365 days prior to randomization, with an interval between the two homologous doses of ≥ 4 weeks to ≤ 12 weeks, and did not receive any other investigational or approved COVID-19 vaccines

4. Female participants must:

   1. Be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal;
   2. Or, if of childbearing potential, be abstinent or agree to use medically effective contraception from 14 days before screening to 30 days following the injection of study intervention. Acceptable forms include:

   i.Implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii.Established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c.Have a negative pregnancy test

5. Participant is willing and able to comply with all required study visits and follow-up required by this protocol.

6. Participant, and the participant's legal representative if applicable, must understand the procedures of the study and provide written informed consent.

Exclusion Criteria:

1. Pregnant or breast feeding or have plan to become pregnant in 30 days after the administration of study intervention.

2. Employees at the investigator's site, of the Sponsor or delegate (e.g., contract research organization) who are directly involved in the conduct of the study.

3. Currently receiving or received any investigational intervention within 30 days prior to the vaccination of study intervention.

4. Administered any licensed live-attenuated vaccines within 28 days or other licensed non-live-attenuated vaccines within 7 days prior to vaccination of study intervention.

5. Administered any blood product or intravenous immunoglobulin administration within 12 weeks prior to the vaccination of study intervention.

6. Currently receiving or anticipated to receive concomitant immunosuppressive or immune-modifying therapy (excluding inhaled, topical skin and/or eye drop-containing corticosteroids, low-dose methotrexate, or < 2 weeks of daily receipt of prednisone less than 20 mg or equivalent) within 12 weeks prior to the vaccination of study intervention.

7. Currently receiving or anticipated to receive treatment with tumor necrosis factor (TNF)-α inhibitors, e.g. infliximab, adalimumab, etanercept within 12 weeks prior to the vaccination of study intervention.

8. Major surgery or any radiation therapy within 12 weeks prior to the vaccination of study intervention.

9. Immunosuppressive illness or immunodeficient state, including hematologic malignancy, history of solid organ, bone marrow transplantation, or asplenia.

10. A history of malignancy with potential risk for recurrence after curative treatment, or current diagnosis of or treatment for cancer (exceptions are squamous and basal cell carcinomas of the skin and treated uterine cervical carcinoma in situ, at the discretion of the investigator).

11. Bleeding disorder considered a contraindication to intramuscular injection or phlebotomy.

12. A history of cerebral venous sinus thrombosis, heparin-induced thrombocytopenia, thrombosis with thrombocytopenia syndrome (TTS), antiphospholipid syndrome, capillary leak syndrome, myocarditis, or pericarditis

13. Participant with ongoing acute diseases or serious medical conditions which will interfere with adherence to study requirements, or the evaluation of any study endpoint.

    Acute diseases or serious medical conditions include cardiovascular, pulmonary, hepatic, neurologic, metabolic, renal, psychiatric condition (e.g. alcoholism, drug abuse, anorexia or severe depression), current severe infections, autoimmune disease, medical history, physical findings, or laboratory abnormality that in the investigators' opinion are not in stable condition and participating in the study could adversely affect the safety of the participant.

14. Documented SARS-CoV1 or 2 infection prior to the study intervention.

15. Participant with a history of hypersensitivity to any vaccine or a history of allergic disease or reactions likely to be exacerbated by any component of the AZD1222, mRNA-1273 or MVC-COV1901.

16. Body (oral, rectal, or ear) temperature ≥ 38.0°C or acute illness (not including minor illnesses such as diarrhea or mild upper respiratory tract infection at the discretion of the investigator) within 2 days before the vaccination of study intervention.

17. Any condition that is a contraindication to study intervention based on the judgement of the investigator